This invention relates to an improved annulus pressure responsive tester valve for use in oil and gas wells. This invention is particularly useful in the testing of offshore wells where it is desirable to conduct testing operations and well stimulation operations utilizing the testing string tools with a minimum of testing string manipulation, and preferably with the blowout preventers closed during most operations.
It is known in the art that tester valves and sampler valves for use in oil and gas wells may be operated by applying pressure increases to the fluid in the annulus between the wellbore and testing string therein of a well. For instance, U.S. Pat. No. 3,664,415 to Wray et al discloses a sampler valve which is operated by applying annulus pressure increases against a piston in opposition to a predetermined charge of inert gas. When the annulus pressure overcomes the gas pressure, the piston moves to open a sampler valve thereby allowing formation fluid to flow into a sample chamber contained within the tool, and into the testing string facilitating production measurements and testing.
In U.S. Pat. No. 3,858,649 to Holden et al a tester valve is described which is opened and closed by applying pressure changes to the fluid in the annulus contained between the wellbore and testing string therein of a well. The tester valve contains a supplementing means wherein the inert gas pressure is supplemented by the hydrostatic pressure of the fluid in the annulus contained between the wellbore and testing string therein as the testing string is lowered into the well. This feature allows the use of lower inert gas pressure at the surface and provides that the gas pressure will automatically be adjusted in accordance with the hydrostatic pressure and environment at the testing depth, thereby avoiding complicated gas pressure calculations required by earlier devices for proper operation. The tester valve described in U.S. Pat. No. 3,856,085 to Holden et al likewise provides a supplementing means for the inert gas pressure in a full opening testing apparatus.
This supplementing means includes a floating piston exposed on one side to the inert gas pressure and on the other side to the annulus fluid pressure in order that the annulus fluid pressure can act on the inert gas pressure. The system is balanced to hold the valve in its normal position until the testing depth is reached. Upon reaching the testing depth, the floating piston is isolated from the annulus fluid pressure so that subsequent changes in the annulus pressure will operate the particular valve concerned.
This method of isolating the floating piston has been to close the flow channel from the annulus contained between the wellbore and testing string in a well to the floating piston with a valve which closes upon the addition of weight to the testing string. This is done by setting the testing string down on a packer which supports the testing string and isolates the formation in the well which is to be tested during the test. The apparatus, which is utilized to isolate the floating piston is designed to prevent the isolation valve from closing prematurely due to increasingly higher pressures as the testing string is lowered into the well, contains means to transmit the motion necessary to actuate the packer and is designed to remain open until sufficient weight is set down on the packer to prevent premature isolation of the gas pressure and thus premature operation of the tester valve.
However, since the tester valve described in U.S. Pat. No. 3,856,085 contains a weight operated tester valve, the tester valve may inadvertently open when being run into the well on a testing string, if a bridge is encountered in the wellbore thereby allowing the weight of the testing string to be supported by the tester valve. Also, in this connection, in highly deviated wellbores it may not be possible to apply sufficient weight to the testing string to actuate the isolation valve portion of the tester valve thereby causing the tester valve to be inoperable. Furthermore, if it is desired to utilize a slip joint in the testing string, unless weight is constantly applied to the slip joint to collapse the same, the isolation valve portion of the tester valve will open thereby causing the tester valve to close.
In U.S. Pat. No. 3,976,136 to Farley et al a tester valve is described which is opened and closed by applying pressure changes to the fluid in the annulus contained between the wellbore and testing string therein of a well and which contains a supplementing means wherein the inert gas pressure is supplemented by the hydrostatic pressure of the fluid in the annulus contained between the wellbore and testing string therein as the testing string is lowered into the well. This tester valve utilizes a method for isolating the gas pressure from the annulus fluid pressure which is responsive to an increase in the annulus fluid pressure above a reference pressure wherein the operating force of the tool is supplied by the pressure of a gas in an inert gas chamber in the tool. The reference pressure used is the pressure which is present in the annulus at the time a wellbore sealing packet is set to isolate one portion of the wellbore from another.
The annulus fluid pressure is allowed to communicate with the interior bore of this tester valve as the testing string is lowered in the wellbore and is trapped as the reference pressure when the packer seals off the wellbore thereby isolating the formation in the well which is to be tested. Subsequent increases in the well annulus pressure above the reference pressure activates a pressure response valve to isolate the inert gas pressure from the well annulus fluid pressure. Additional pressure increases in the well annulus causes the tester valve to operate in the conventional manner.
Once a well has been tested to determine the contents of the various formations therein, it may be necessary to stimulate the various formations to increase their production of formation fluids. Common ways of stimulating formations involve pumping acid into the formations to increase the formation permeability or hydraulic fracturing of the formation to increase the permeability thereof or both.
After the testing of a well, in many instances, it is highly desirable to leave the testing string in place in the well and stimulate the various formations of the well by pumping acids and other fluids into the formations through the testing string to avoid unnecessary delay by pulling the testing string and substituting therefore a tubing string.
During well stimulation operations in locations during extremely cold environmental periods where the tester valves described in U.S. Pat. Nos. 3,856,085 and 3,976,136 are utilizing in the testing string if large volumes of cold fluids are pumped through the tester valves, even though the formations surrounding the tester valves may have a temperature of several hundred degrees fahrenheit, the tester valves will be cooled to a temperature substantially lower than the surrounding formations by the cold fluids being pumped therethrough. When these tester valves are cooled by the cold fluids, the inert gas in the valves contracts. Upon the cessation of the pumping of cold fluids through the tester valve, if it is desired to close the test valve by releasing the fluid pressure in the annulus between the wellbore and testing string, since the inert gas has contracted due to the cooling of the valve, the inert gas in its cooled state may not exert sufficient force to close the tester valve to thereby isolate the formation which has been stimulated from the remainder of the testing string. If this condition occurs, it will be necessary to maintain the fluid pressure in the testing string at the surface thereof and wait for the formation to warm the tester valve until the inert gas expands sufficiently to regain the pressure level required to close the tester valve when the fluid pressure in the annulus between the wellbore and testing string is released. Since this warming of the inert gas can require a lengthy period of time during which the flow from the formation cannot be controlled by the tester valve, an undesirable condition which affects control of the well exists.
While it is theoretically possible to charge the inert gas chambers of the tester valves at the surface to compensate for the cooling effect of pumping cold fluids through the tester valves, if the cooling effect can be ascertained, this would cause the pressure levels of the fluid in the annulus between the wellbore and testing string to be unacceptable when the tester valve is at the temperature of the surrounding formation thereby risking damage to the testing string. Furthermore, in actual practice, compensating for the cooling effect of the tester valve by overcharging of the inert gas chamber at the surface, cannot be accomplished in most instances because the precise cooling effect cannot be easily ascertained due to the unknown heat transfer characteristics of the fluid being pumped through the testing string and the surrounding formations.